| The objective of this study was to characterize restorative dental composite materials exposed to different environmental conditions using confined compression loading and microtomography. Confined compression testing more closely represents the complex loading dental restorations experience in situ. Filtek Supreme and Renew dental composite were prepared as cylindrical specimens (r=2.36 mm, h=3.70 mm) and cured in 6061 aluminum ring molds. Composite specimens were (i) aged in artificial saliva for three months, (ii) cyclically loaded under confined compression between 100-300 N for 100,000 cycles, and/or thermocycled in alternating 15° F/75° F water baths for 2,200, 4,400, 8,800 and 13,000 cycles. The specimens were then subjected to confined compression loading to an axial strain of 12%, which is achieved by multiaxially loading samples in their rings having a confinement of lambda=2, where lambda is the ratio of the ring's outer/inner diameter. Radial expansion of the composite generates confinement stresses. The area under both the axial stress vs. axial strain curve and the confinement stress vs. axial strain curve ("work of strain") was calculated to compare differences between experimental groups.;Tomographic data was generated for each specimen using a high-resolution microtomography system developed at beamline 2-BM of the Advanced Photon Source (APS) at Argonne National Laboratory. Extraction of the crack and void surfaces present in the material bulk are numerically represented as crack density, and calculated as a fraction of total specimen volume.;Aluminum confining rings appear to prevent appreciable cracking within the sample because they constrain the samples excessively. Although differences between experimental groups do appear in both the mechanical results as well as the tomographic image analysis, significant distinctions are not pronounced, likely due to the choice of confining ring material and small sample sizes. However, thermal and mechanical cycling has an effect on the ability of the composite to withstand load as well as to increase cracking density. Use of multiaxial loading in conjunction with microtomography for dental materials is a viable method in the investigation of composite degradation; with further enhancements it can yield valuable insights into early material breakdown in a nondestructive manner. |
